I used my HELOC to buy dividend paying stocks. Dividends are either tax-free or close to it for most Canadians, and interests on HELOC are tax-deductible. Between the two, your after-tax cash flow should only be slightly negative during the first year or two. After a few years of dividend increases, it should turn into a positive and rising cash flow portfolio for many years to come.
It doesn't make sense to wait until your house is paid off before investing in dividend stocks for the same reason that it doesn't make sense to wait until you have enough cash to buy your home outright.
Buying dividend stocks with cash may seem safe, but it's killing your time. The longer you wait, the more time you'll lose. Investing while you young is safe, because you have time horizon on your side. If you're 50 year old by the time you pay off your mortgage, you'll have only a few years left before "early retirement". That is when it's too risky to buy stocks.